In the manufacture of conventional phonographic cartridges, including cartridges utilized in both commercial and domestic phonographic systems, it is typical for one side of the cartridge coil of a stereo system to be connected to ground, thus resulting in unbalanced phonograph preamplifier circuitry. The unbalanced nature of this preamplifier circuit results in the reception of a certain amount of noise which must be compensated for by the provision of appropriate filtering circuits. In the case of domestic phonograph systems, very little filtering is typically accomplished and therefore domestic phonograph cartridge systems are subject to hum and noise. Phonographic cartridges produce very low level outputs and thus the leads which connect a cartridge to the accompanying preamplifier are very susceptible to hum and noise pick-up. In high-fidelity, low-noise applications, it has traditionally been necessary to minimize the length of these leads and to route the leads carefully so as to reduce hum pick-up from nearby radiating alternating current fields. Accordingly, preamplifiers are mounted as near as possible to phonographic cartridges, often in awkward, hard to service locations, with unpredictable results.
Microphones also have low-level outputs, but microphone preamplifiers are generally designed to eliminate hum pick-up through the use of balanced-input circuitry, which electronically cancel any noise that is induced on both leads coming from the microphone. Tradiationally, an input transformer has been utilized as the input balancing component. Because common phonographic cartridges, unlike microphones, must be terminated by high-impedance, low-inductance loads, input transformers are impractical for use in phonographic preamplifiers.
The development of new low-noise transistors has promoted the use of transformerless, balanced-input circuits as replacements for transformers in microphone preamplifier designs. Such designs use differential amplifiers to eliminate input-noise. Differential amplifiers have virtually no inductance and can be designed with the proper input impedance to load phonographic cartridges, making possible their use in phonographic preamplifiers.
In view of the foregoing, it is a primary feature of the present invention to provide a novel, balanced phonographic preamplifier circuit that eliminates the susceptibility to noise and hum that is often associated with unbalanced phonographic preamplifier circuits.
It is another feature of this invention to provide a novel balanced-input preamplifier circuit for phonographs which can be efficiently employed with either balanced or unbalanced input signals.
It is another feature of this invention to provide a novel balanced-input preamplifier circuit for phonographs that effectively eliminates any radio frequency noise even under circumstances, such as in radio stations, wherein the phonograph system is located near a radio transmitter and might otherwise be susceptible to transmitter noise.
It is an even further feature of this invention to provide a novel, balanced-input preamplifier circuit wherein a feedback loop is employed that is substantially isolated from the phonographic cartridge.
Among the several objects and features of this invention is contemplated the provision of a novel, balanced-input preamplifier circuit for phonograph systems that is equally applicable to phonographic preamplifiers having variable gain controls or variable equalization controls.
Other and further objects, advantages and features of this invention will become obvious to one skilled in the art upon an understanding of the illustrative embodiment about to be described and various advantages, not referred to herein, will occur to one skilled in the art upon employment of the invention in practice.